Engine systems may use a diesel particulate filter (DPF) to reduce particulate emissions. In the case of an active DPF, deliberate changes to vehicle operation may be used to regenerate the filter, and may be triggered when the particulate matter builds up to levels where pressure drop across the filter exceeds a specified level. Active regeneration of a loaded DPF can take several tens of minutes and may require that the exhaust gas supplied to the DPF has a considerable excess of oxygen (over reductants) to ensure sufficient oxygen to sustain combustion of the particulate matter. Engine systems may also include lean NOx traps (LNT) used to control NOx emissions. Lean NOx traps may also variously be referred to as NOx storage-reduction catalysts, or simply NOx catalysts.
One approach for particulate filter regeneration in a system also having a NOx catalyst is described in 2004/0123590. In this approach, during DPF regeneration, the air-fuel ratio is periodically made rich to reduce sulfur poisoning of the NOx catalyst.
However, the inventors herein have recognized a potential disadvantage with such an approach. In particular, catalyst deactivation of the NOx catalyst may occur during the high temperature lean operation of DPF regeneration. Further, the rich operation for sulfur reduction occurring during the high temperature DPF regeneration may not last for sufficient duration (or may not be rich enough) to reactivate the NOx catalyst. Moreover, the periodic rich operation may be terminated during the lean portion of the cycle, thus leaving the NOx catalyst in the deactivated state. Thus, NOx performance may degrade even though the sulfur has been removed, as the NOx catalyst may be made substantially inactive due to high temperature lean operation, separate and apart from, sulfur accumulation.
In one approach, the above issues may be at least partially addressed by, after lean DPF regeneration, providing a rich or intermittently rich exhaust air-fuel ratio for a duration lasting until NOx catalyst temperature falls to a minimum threshold. Such operation may be used to reactivate NOx catalysts and reverse the deactivation that may have occurred from the high temperature lean operation associated with the previous DPF regeneration. In one particular example, such operation may be provided in response to an engine shut-down during DPF regeneration, thereby enabling improved performance on subsequent engine starts.